Nitrite production by thermophilic aerobic bacteria isolated during the manufacture of milk powder

Nitrite contamination in milk powder is hypothesised to be caused by thermophilic bacteria that form biofilms in milk powder manufacturing plants. Regulatory limits in some countries have made nitrite contamination in milk powder an issue influencing product sales. In this study, thermophilic bacteria were isolated from milk samples obtained from a milk processing plant to investigate the potential of different isolates to convert nitrate to nitrite. Eight bacteria species were identified through 16s rDNA gene sequencing (Bacillus licheniformis, Bacillus aerius, Bacillus subtilis, Bacillus sonorensis, Bacillus firmus, Enterococcus faecium, Geobacillus stearothermophilus and Macrococcus caseolyticus). Most bacteria isolated were able to convert nitrate to nitrite, and surprisingly in aerobic conditions. The most interesting species was G. stearothermophilus as its nitrate reducing capability was highly variable between different isolates. This could explain why there was no relationship reported between nitrite level in milk powder and high thermophile count.